At present, exhaust gases of internal combustion engines still contribute to the air pollution although energetic endeavors have been made to solve this problem. Catalytic conversion of noxious or harmful components of the exhaust gases is considered as one of promising ways to the solution, so that various catalysts have been proposed and partly are in practical use. Most of the catalysts developed until now are only capable of promoting either the oxidation of hydrocarbons and carbon monoxide or the reduction of nitrogen oxides.
However, some recently developed catalysts are active simultaneously on the oxidation of hydrocarbons and carbon monoxide and on the reduction of nitrogen oxides if an engine exhaust gas at the entrance to a catalyst column contains oxidizing components and reducing components within a certain proportion range. In general, a catalyst of this type consists of a combination of two or more kinds of noble metals, particularly of platinum group metals exemplified by a combination of platinum and rhodium, applied to a carrier of an active or transition form alumina. As is commonly accepted, "active (or transition form) alumina" is the general term for gamma-, delta-, eta-, theta-, iota-, chi- and kappa-alumina including their mixtures. Accordingly, alpha-alumina and certain alumina hydrates are excluded from this definition. The catalysts of this type have satisfactorily good initial activity but are devoid of the ability of retaining their activity on continual exposure to engine exhaust gases for a long period of time. When these catalysts are used in automobiles, their activity significantly lowers after 10000-20000 km of vehicle operation. Accordingly these catalysts are not yet fully practicable.
A somewhat different catalyst is described in "J. Chem. Soc. of Japan", Vol. 84, No. 9 (1963), p. 17 as an acidic cracking catalyst. In the reported catalyst, platinum is applied to a carrier which contains 10-20% boria and is produced by impregnating alumina with boric acid and firing the impregnated alumina. We have examined the applicability of this catalyst to the conversion of engine exhaust gases with the result that the catalyst has not sufficiently high activity on the simultaneous conversion of hydrocarbons, carbon monoxide and nitrogen oxides and is very poor in the ability to retain its activity on continual exposure to engine exhaust gases.